First question would be where do you want to upload TO?
First, you have to be sure you're in the subdomain you want to publish to. Either use the tab menu towards the top, or the jump menu in your user panel.
From there, you would click the "add story" link. Once you've filled out your title, summary, and selected any appropriate warnings (all required), select your primary category. I don't know why, but it's not allowing selection of subcategories at the moment. We will look into this.
Unfortunately, there appears to be a bug, in that after you do all this, and then publish, you must then go back and EDIT to make sure it's in the right place. So, after you publish the story initially, for now, until this is fixed, you then would edit, and then select the primary category, and then the sub or sub sub. The sub sub categories appear in the subcategory drop down with their parent, followed by ">" and the sub sub name.